Human pluripotent stem cells (hPSCs) are a useful tool to generate any adult cell type for transplantation into patients. In principle, hPSC-based cell therapies have the potential to treat most if not all degenerative illnesses, however the success of such therapies may be limited by a subject's immune response.
The immune system protects organisms from infection with layered defenses of increasing specificity. In simple terms, physical barriers prevent pathogens such as bacteria and viruses from entering the organism. If a pathogen breaches these barriers, the innate immune system provides an immediate, but non-specific response. If pathogens successfully evade the innate response, vertebrates possess a second layer of protection, the adaptive immune system, which is activated by the innate response. The adaptive immune system generates a much more specific response. Here, the immune system adapts its response during an infection to improve its recognition of the pathogen. This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory, and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered The adaptive immune response is antigen-specific and requires the recognition of specific “non-self” antigens during a process called antigen presentation. Antigen specificity allows for the generation of responses that are tailored to specific pathogens or pathogen-infected cells. Interferon gamma (IFN-γ) plays an essential role in combating infectious and non-infectious diseases. The principal source of IFN-γ in the human immune response is T cells. NK cells, macrophages, and IFN-play an important role in both innate and acquired immunity.
The major histocompatibility complex (MHC) is a set of cell surface proteins essential for the regulation of the immune system. The main function of MHC molecules is to bind to antigens derived from pathogens and display them on the cell surface for recognition by the appropriate T-cells. The MHC gene family is divided into three subgroups: class I, class II, and class III. The human MHC is also called the HLA (human leukocyte antigen) complex (often just the HLA).
Natural killer (NK) cells are lymphocytes that function at the interface between innate and adaptive immunity. NK cells contribute directly to immune defense through their effector functions, such as cytotoxicity and cytokine secretion, and by regulating innate and adaptive immune responses. When a target or host cell encounters NK cells several outcomes are possible. The extent of the NK response is determined by the amount and type of activating and inhibitory receptors on the NK cells and the amount and type of activating and inhibitory ligands on the target cell. See FIG. 1. In scenario A, when target cells have no human leucocyte antigen (HLA) Class I and no NK activating ligands, NK cells expressing MHC-Class I inhibitory receptors and activating ligand receptors do not attack target cells (no response, or not-licensed). In scenario B, when target cells express HLA-Class I but have no activating ligands, the NK cells expressing inhibitory receptors and activating receptors cannot attack the targets. In scenario C, when target cells have downregulated HLA-Class I or no HLA-Class I and express NK activating ligands, NK cells expressing inhibitory receptors and activating receptors attack target cells. In scenario D, when target cells express both self-HLA-Class I and NK activating ligands, then the level of response by NK cells expressing inhibitory receptors and activating receptors is determined by the balance of inhibitory and activating signals to the NK cell. Haynes et al., THE IMMUNE SYSTEM IN HEALTH AND DISEASE, PART 15: Immune-Mediated, Inflammatory, and Rheumatologic Disorders, 372e Introduction to the Immune System.
Historically, efforts to overcome a host's immune response to allogenic cells focused on the adaptive immune response, that is, interfering with adhesion between T-cells and MHC-Class I antigens presented on foreign cells. As such, CRISPR and TALEN systems have been used to generate loss of function genetic modifications and thus make stem cells that do not express one or more classic MHC/HLA genes. However, these cells and cells derived therefrom are still vulnerable to the host's innate immune response (NK cells). See, e.g., Parham et al. (2005) Nat Rev Immunol. 5(3):201-214. In order to overcome the host's innate immune response, others have tried to reintroduce tolerogenic factors back into the target cell; the focus was on the “missing self.” See WO2016183041A2 the disclosure of which is incorporated by reference in its entirety. Applicants surprisingly discovered that the key to evading the host's NK mediated immune response is not the “missing self” but the expression and magnitude of NK cell activating ligands on target cells.
Thus, there remains a need for compositions and methods for developing target cells that lack some or all classic HLA expression but which cells are not attacked by NK cells for lysis.